An example of conventional ring shaping apparatus is disclosed in U.S. Pat. No. 3,803,890.
This apparatus comprises a mandrel disposed centrally of the apparatus for extending through a workpiece, four rolls for rotatably supporting the workpiece at outer peripheral positions thereof, and two shaping rolls one of which presses against the other to roll out the workpiece.
This known apparatus, however, has the disadvantage of necessitating a separate, complicated mechanism for rotating the mandrel reliably.
Furthermore, the known apparatus has the drawbacks of providing low rolling efficiency and having difficulties in shaping the workpiece with a high degree of roundness.
U.S. Pat. No. 1,720,833 discloses another example of ring shaping apparatus which includes a pair of backup rollers for rotatable pressure contact with peripheral surfaces of a workpiece. These backup rollers, however, are manually and individually controlled by means of handwheels, and the two backup rollers tend to apply unequal pressing forces to the workpiece. It is therefore impossible to shape the workpiece with a high degree of roundness. With this apparatus, a shaping operation must be followed by a sizing operation for finishing either the inside or outside of the workpiece to agree with a required size. The sizing operation involves press-fitting of the workpiece with a mold and, for processing a large workpiece, requires a large sizing apparatus. This gives rise to a problem of low productivity.